docs: add code samples for ml.metrics.r2_score (#459)

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Fixes internal #330220122 🦕

Author: ashleyxuu

third_party/bigframes_vendored/sklearn/metrics/_regression.py

@@ -42,6 +42,18 @@ def r2_score(y_true, y_pred, force_finite=True) -> float:
     these cases are replaced with 1.0 (perfect predictions) or 0.0 (imperfect
     predictions) respectively.
 
+    **Examples:**
+
+        >>> import bigframes.pandas as bpd
+        >>> import bigframes.ml.metrics
+        >>> bpd.options.display.progress_bar = None
+
+        >>> y_true = bpd.DataFrame([3, -0.5, 2, 7])
+        >>> y_pred = bpd.DataFrame([2.5, 0.0, 2, 8])
+        >>> r2_score = bigframes.ml.metrics.r2_score(y_true, y_pred)
+        >>> r2_score
+        0.9486081370449679
+
     Args:
         y_true (Series or DataFrame of shape (n_samples,)):
             Ground truth (correct) target values.